This invention relates to novel compositions of matter useful as lithographic photosensitizers. More specifically, the present invention relates to novel lithographically suitable diazo photosensitizers useful in the manufacture of color proofing guides. Still more particularly, the instant invention relates to novel lithographic photosensitizers which when blended with a suitable dye and a compatible resin in an acceptable solvent system, form compositions which when applied to transparent substrates form commercially satisfactory color proofing guides for use by lithographers. The photosensitizers of the instant invention are beneficial in that they are substantially developable with water.
The art of producing good quality color lithographic reproduction depends upon a multitude of factors, one of which is the selection of the proper tones to match the colors of the original image. It has, therefore, been found highly desirable to provide the lithographer with an inexpensive means for comparing the image which he intends to reproduce with the original image before he has begun a press run. To this end it is known to manufacture color proofing guides which the lithographer can use a priori to evaluate the quality of his image and make any required adjustments before beginning production. Typically, these color proofing guides are comprised of a photosensitive composition and a dye, usually having a cyan, magenta, yellow or black color, which has been coated upon a transparent substrate. Upon exposure of the color proofing guide to the original image and development by standard techniques known to the skilled worker, a transparent image is produced which corresponds to one of the colors of the original image. A color proofing guide is made for each of the above mentioned primary colors to produce a series of transparencies each possessing one color of the original image. When these transparencies are superimposed an image is formed which closely approximates that of the original image.
The lithographer may then compare his proof image with that of the original image and make any adjustments necessary to more closely match the original image before proceeding with actual printing. Such changes which can be made include altering exposures and halftone patterns.
A problem with this method has been that the photosensitive compositions typically employed in the manufacture of these color proofing guides have required development using environmentally undesirable solvents, caustics and other similar ingredients. It has long been desired to produce such a color proofing guide which is developable to a commercially acceptable extent using common tapwater. Unfortunately, this has not been possible heretofore. Severe development problems and poor image quality have been noted when those skilled in the art have attempted to produce such images by developing with water.
The present invention provides a photosensitive composition useful for color proofing guides which are easily developable with water, demonstrate good wet adhesion and have minimal background staining even after the composition has been heated for drying purposes during the manufacturing process. The composition employs two diazo compounds, the combination of which demonstrates the aforementioned benefits even though the individual diazo compounds do not have such beneficial properties.